If you've ever watched a group of puppies play, you know they can really
roughhouse at times. This can include a number of behaviors, including
mouthing and nipping, which are separate and distinct.

Puppy mouthing is a more gentle form of placing their teeth and may be a
prelude to nipping, depending on the pup's level of excitement.
Generally, it is one of the ways a puppy explores its world and it may
also be because the pup is teething. If your puppy is teething (which
generally happens anywhere between the ages of 2 and 8 months), be sure
and offer plenty of OK things to chew on such as ice cubes in warmer
weather, an old washcloth that you've put in the freezer or a soft ball
or toy. It's probably a good idea to avoid the harder chews during
teething, such as rawhide.

If you don't want your pup to mouth you, keep a substitute chew
available at all times. If your young dog starts mouthing, say "no"
firmly and then immediately offer something that's acceptable to put in
her mouth. You may need to push her head away as well if she doesn't
give up. When she takes the proffered toy or other "chewy" praise her
warmly and smile!

Nipping is the next stage past mouthing and many herding breeds such as
the Australian Shepherd and Border Collie do this naturally. Your goal
is to teach your puppy that putting any pressure on your skin or
clothing with their sharp teeth is unacceptable.
Start by doing what other dogs do when a pup nips - yelp loudly! - as
this sound indicates displeasure. Then you can do one of several things:
Begin by putting your pup in a time out for 5 to 10 minutes with no
physical contact with you. Let him out and if he nips again, say "no
nipping" or "no biting" in a louder than usual voice and shake him a
couple of times by the scruff of the neck. (Do not do this too roughly -
you are warning your pup to stop and this is often what adult dogs do.
The goal is not to hurt but to warn.) You may want to follow this with
another time out or even a growling sound to let your pup know that you
really are upset. Repeat as necessary, being consistent with your words
and actions.

Make sure that the nipping is not just the result of boredom or lack or
regular daily exercise. Puppies sleep a lot but when they wake up, they
are raring to go! Engage in some fun play every day with your pup such
as hide and seek, and you can also divert their attention by starting to
work on some simple training techniques. Fetch might be a good place to
start if your young dog loves to place things in her mouth. But
training of any kind (sit, come, stay) will help her re-focus her
attention elsewhere.

Most unwanted canine behaviors can be corrected with patience and
consistency. Be firm, not rough in setting your boundaries and make sure
your pup gets what he needs: good food, fun, training, love and
exercise. Your investment of time early on will reap benefits for the
rest of his life!